What are non-CO2 effects of aviation?
On top of CO2, aircraft engines emit other gases – nitrous oxides (NOx), sulfur dioxide (SO2) and water (H2O) – and particulate matter (soot). When emitted at high altitudes, these emissions affect atmospheric physical and chemical properties. This results in an increase in greenhouse gases, and the potential formation of persistent contrail cirrus. These high, linear clouds trap the Earth’s heat.
The consequence is a net warming effect, which may be up to three times worse than the warming caused by aviation’s CO2 emissions.
Nevertheless, these effects are also short lived, meaning that acting against them would quickly reduce aviation’s contribution to global warming, scoring important wins in our fight against climate change.
How can we mitigate the non-CO2 impacts of flying?
Persistent contrail formation is mainly the result of soot and other emissions on cold, high humidity atmospheric areas known as Ice Super Saturated Regions (ISSRs). But solutions exist to tackle the problem.
To reduce non-CO2 effects, airplanes can use clean fuels to reduce the amount of pollutants released in the air. Jet fuels with high aromatics and naphthalene concentrations increase soot formation, which in turn leads to persistent contrail cirrus. One way to reduce aromatics and naphthalene in jet fuel is to perform hydrotreating on fossil jet fuels. Reducing aromatics content of fossil jet fuel down to 8-10% can be achieved without significant costs and could lower non-CO2 effects significantly.
Avoiding flying through areas with very cold and humid conditions, known as Ice Super Saturated Regions (ISSR), is a key axis to reduce non-CO2 effects. Changing flight paths to fly at a lower altitude, or performing small diversions, can avoid contrail formation. For instance, rerouting less than 2% of flights in Japan can reduce the warming effect of contrails by 60%.
In order for researchers to better predict which regions should be avoided, airlines must cooperate with the EU and make the data collected on their flight routes available.
What is being done?
The first piece of evidence highlighting the importance of aviation’s non-CO2 effects came from the UN’s Intergovernmental Panel on Climate Change (IPCC) back in 1999. Since then, a lot of excellent research has been carried out to better understand the effects of these emissions and how to tackle them.
The European Commission was first tasked with addressing the non-CO2 emissions of flying in 2008, and commissioned a landmark report to the European Union Aviation Safety Agency (EASA). The report, published in 2020, analyzed the latest available science, quantified non-CO2 climate impact of aviation as twice that of CO2, and proposed some mitigation measures.
However, these measures – and non-CO2 effects in general – are hardly reflected in EU. The EU is therefore losing valuable time to mitigate most of the climate impact of aviation.